Longitudinal (d33) mode stacked and transverse (d31) mode tube or cylinder actuators are widely used piezoelectric actuators when high generative forces are required. These actuators work fine for isotropic materials, such as lead zirconate titanate (Pb[Zr1-xTix]O3 or PZT) piezo-ceramics.
To keep the applied voltage low, longitudinal (d33) mode actuators are often made up of shorter disk, ring or plate-like segments of a few mm or smaller in thickness bonded together into a stack via a suitable means.
In recent years, lead zinc niobate-lead titanate (Pb[Zn1/3Nb2/3]O3—PbTiO3 or PZN-PT), lead magnesium niobate-lead titanate (Pb[Mg1/3Nb2/3]O3—PbTiO3 PMN-PT) and lead magnesium niobate-lead zirconate titanate (PMN-PZT) solid solution piezoelectric single crystals and their doped and/or modified ternary and quaternary derivatives have been investigated extensively. These new piezoelectric materials exhibit superior electro-mechanical properties to PZT ceramics.
Piezoelectric single crystals are anisotropic materials which exhibit different properties when poled and/or activated in different crystal directions.
For instance, for a [001]-poled longitudinal (d33) mode single crystal ring, while it exhibits fairly uniform longitudinal strain and sound velocity in the axial direction across the top and bottom faces, its strains and sound velocities vary with different radial directions. As a result, a short single crystal ring often exhibits complex coupled resonant behavior and may not be suitable for operating in resonant mode.
Reference made to Journal of the Acoustical Society of America, vol. 121, pp. 2591-2599 (2007) entitled “Single-crystal lead magnesium niobate-lead titanate (PMN/PT) as a broadband high power transduction material”, by Moffett et al. wherein a single crystal cylinder projector made up of 9 segmented-rings stacked and bonded on top of one another, each ring being a dodecagon of 12 beveled d33-mode single crystal plates is disclosed.
Reference made to “Single crystal free-flooded ring transducer”, presented at 2010 U.S. Navy Workshop on Acoustic Transduction Materials and Devices, May 11-13, State College, Pa., USA. (Abstract No. IV.1) (2010) by Robinson et al. [22] wherein a hybrid transducer in which wedge-shaped spacers made of alumina were used as bonding guides and stiffeners to bond rectangular d31-mode single crystal active elements into segmented rings which were then stacked and bonded together to form the cylinder projector is reported.
In the above two cited documents, the actuation mode of interest is the circumferential vibration or breathing mode of the ring or cylinder.
Single crystal tube actuators of transverse mode (either d31 or d32) also suffer from problems arising from crystal anisotropy. Such actuators typically have electrodes in the inner and outer surfaces of the tube and are poled in the radial direction. Since the radial directions of a single crystal tube constitute different crystal directions, when the single crystal tube is put into action with the applied electric field in the radial direction, non-uniform strains in the axial direction will be generated at both end faces of the tube. This is undesirable because the overall performance of the tube actuator is compromised by the radial segments exhibiting lower performance than expected.
Another major disadvantage of single crystal rings and tubes is that they have to be machined out from a block of crystal and the material wastage involved in such a process can be 50% or even higher. This increases the already high cost of production of piezoelectric single crystal further.
The machining of small-diameter single crystal tubes is especially troublesome which often results in high rejection rates. This is because the curved machined surfaces are not easy to dress by mechanical polishing to remove machining induced defects. These cracks, if not removed, may propagate during poling of the crystal. And, even if the actuator survives the poling process, the cracks may grow during use, leading to premature failure of the device.
Reference made to “Naval device applications of relaxor piezoelectric single crystals”, in Proceedings of the 2002 IEEE Ultrasonics Symposium, pp. 733-737 (2002) by Rehrig et al wherein an underwater transducer driven by multiple single crystal blocks as the motor section was reported, in which no edge stiffeners was used. Such a design works fine when the single crystals active elements are short, thick and strong but would be problematic when the crystals are long and thin and the device is subject to bending and twisting as would be expected in typical application of most piezoelectric actuators.
It is, therefore, a motivation of the present invention to provide a multi-stake actuators made of single crystal active elements which obviates the drawback of the prior art.